When a fluorinated polymer such as polytetrafluoroethylene (hereinafter referred to as PTFE) is formed by using an emulsion polymerization method, a fluorinated emulsifier is usually used, since it does not hinder the polymerization reaction of a fluorinated monomer by chain transfer, in an aqueous medium.
An aqueous PTFE emulsion is obtained by emulsion polymerization of tetrafluoroethylene (hereinafter referred to as TFE). By coagulating the aqueous emulsion, followed by drying, a PTFE fine powder is obtained. The fine powder is shaped by paste extrusion, and then, is used for various purposes. On the other hand, an aqueous PTFE dispersion obtained by having the aqueous emulsion itself concentrated or treated for stability, as the case requires, is used for various applications such as an application for coating, or an application for impregnation by adding various additives.
For emulsion polymerization of a fluorinated monomer, ammonium perfluorooctanoate (structural formula: CF3(CF2)6COONH4, hereinafter referred to as APFO) is usually used, as a fluorinated emulsifier. In recent years, from an environmental viewpoint, it has been proposed to control disposal of APFO, since APFO is not present in nature and is hardly decomposed. Further, it has been pointed out that APFO has a high bioaccumulation potential.
However, in emulsion polymerization of TFE, it is difficult to decrease the amount of the fluorinated emulsifier to be used. Further, in order to recover the fluorinated emulsifier from the aqueous PTFE emulsion, the aqueous PTFE dispersion or the wastewater from coagulation after emulsion polymerization, a new additional facility will be necessary, and the cost of making PTFE will be increased.
Heretofore, a fluorinated emulsifier other than APFO, has been proposed which is used for copolymerization of a fluorinated monomer alone or a fluorinated monomer and another monomer (Patent Documents 1, 2, 3 and 4).
Patent Document 1 discloses Examples wherein a fluorinated emulsifier of the formula F—(CF2)p—O—(CF(Y)—CF2—O—)q—CF(Y)COOB wherein Y is a fluorine atom or a perfluomethyl group, p is from 1 to 5, q is from 0 to 10, and B is a hydrogen atom or a monovalent salt, is used. Among the Examples, there is a case wherein a fluorinated emulsifier of the structural formula CF3CF2OCF2CF2OCF2COONH4 is used to copolymerize TFE and hexafluoropropylene (hereinafter referred to as HFP). In this case, the amount of the fluorinated emulsifier used is calculated to be about 1,040 ppm, based on the amount of finally formed copolymer.
However, Patent Document 1 discloses no case of polymerization of TFE alone by using the fluorinated emulsifier. Further, when TFE was polymerized alone by using the fluorinated emulsifier in an amount of 1,040 ppm, based on the amount of finally formed PTFE (hereinafter referred to also as the final yield of PTFE), the polymerization speed showed an abnormal rate, and a large amount of a solidified product was formed, whereby it has been found difficult to carry out the emulsion polymerization stably.
Further, the standard specific gravity of a copolymer of TFE and HFP, which was obtained by the Example of Patent Document 1, is 2.220. This value of the standard specific gravity indicates that the molecular weight of the copolymer is low. Namely, it is indicated that there is a problem such that even if TFE and HFP are copolymerized by using the fluorinated emulsifier, it is possible only to obtain a TFE/HFP copolymer having a low molecular weight.
Further, the copolymer of TFE and HFP in an aqueous emulsion, which is obtained by the Example in Patent Document 1, has an average primary particle size of 0.176 μm, i.e. the particle size is small. In general, in a case where paste extrusion is carried out by using a fine powder obtained from particles having a small average primary particle size, there is a problem of insufficiency of extrusion processability such that the pressure of paste extrusion increases, and the appearance of the shaped product is impaired. Further, in a case where an aqueous PTFE dispersion obtained by adding various additives to an aqueous PTFE emulsion, is used in an application for coating, there will be a problem such that if the average primary particle size is small, cracks are likely to form on the coated film.
Further, in emulsion polymerization of TFE, it is generally known that if the amount of the fluorinated emulsion used is increased, the average primary particle size of PTFE to be obtained becomes small. In the case of the Example in Patent Document 1, it is anticipated that if the amount of the fluorinated emulsifier used is increased, the average primary particle size would be smaller.
In an Example in Patent Document 2, CF3CF2CF2C(CF3)CF2OCF(CF3)COONH4 is disclosed, as an emulsifier for polymerization, and it is found that the emulsifier has a higher bioaccumulation potential than APFO.
In Examples in Patent Document 3, CF3CF2CF2C(CF3)2(CH2)2COONH4, etc. are disclosed, as emulsifiers for polymerization. In general, in a fluorinated emulsifier, if hydrogen atoms are introduced into the molecule of the fluorinated emulsifier, chain transfer is likely to take place during the polymerization of a fluorinated monomer, whereby there will be a problem such that the molecular weight of a fluorinated polymer to be obtained will not be sufficiently high.
In Examples in Patent Document 4, F(CF2)5OCF(CF3)COONH4, etc. are disclosed, as emulsifiers for polymerization.
Patent Document 1: JP-B-39-24263
Patent Document 2: JP-A-2003-119204
Patent Document 3: JP-A-2002-308914
Patent Document 4: JP-A-2002-317003